The construction of optical transmission networks based on wavelength division multiplexing (WDM) technology has been advanced, and optical add-drop multiplexers (OADM) capable of inserting (adding) and branching (dropping) light included in WDM light in units of wavelength have been actively developed and introduced.
In recent years, flexible grid technology that enables arbitrarily setting of wavelength intervals and optimization of frequency usage efficiency, and signal speed increase technology using waveform shaping processing or the like by digital signal processing have been studied.
For example, the development of optical transmission apparatuses that support transmission speed of 100 gigabits/second (Gbps) per wavelength have has been advanced. Also, it has been examined to put an optical transmission apparatus that multiplexes a plurality of wavelengths to support larger capacity and higher speed optical transmission, that is, optical transmission at 400 Gbps, 1 terra (T) bps, or the like, into practical use.
Such larger capacity and higher speed optical transmission is occasionally referred to as “super channel” transmission, in contrast to normal WDM transmission. In a “super channel”, while inter-wavelength interference is reduced by using waveform shaping processing by digital signal processing, a wavelength arrangement interval may be reduced to a smaller interval than that of normal WDM.
Therefore, in super channel transmission, it is possible to increase, as compared to normal WDM transmission, usage efficiency for a wavelength (frequency) resource in a pass band. Note that a wavelength included in a super channel is occasionally referred to as a “subcarrier (SC)”. Therefore, super channel transmission may be referred to as “multicarrier transmission”.
As one of technologies used for increasing usage efficiency for a wavelength resource in a pass band, a technology called “flexible grid” is known. In accordance with the flexible grid technology, an interval between adjacent wavelengths may be arbitrarily set (made variable).
Since a wavelength interval may be made variable, in the flexible grid technology, a fragmented vacant wavelength band tends to be generated. Thus, a technology called “wavelength defragmentation” used for putting as many fragmented vacant wavelength bands as possible together by rearrangement (which may be referred to as “wavelength shift”) has been studied.
By putting vacant wavelength bands together, for example, accommodation of a new signal optical wavelength in the vacant wavelength bands may be made easier and thus frequency usage efficiency of a pass band may be increased. Note that the “wavelength defragmentation” may be referred to as “wavelength defrag” or may be abbreviated merely as “defrag”.
Japanese Laid-Open Patent Publication No. 2013-106328 discusses related art.
Also, F. Cugini et al., “Push-Pull Technique for Defragmentation in Flexible Optical Networks”, OFC/NFOEC Technical Digest, 2012, OSA, Roberto Proietti et al., “Rapid and complete hitless defragmentation method using a coherent RX LO with fast wavelength tracking in elastic optical networks”, 19 Nov. 2012, Vol. 20, No. 24/OPTICS EXPRESS pp. 26958-26968, and Kyosuke Sone et al., “First Demonstration of Hitless Spectrum Defragmentation using Real-time Coherent Receivers in Flexible Grid Optical Networks”, in Proc. Of ECOC 2012, paper Th.3.D.1, September 2012 also discuss related art.
In conventional technologies, there has been no study at all conducted to examine a probability in which, when wavelength shift is performed on a wavelength (a subcarrier) included in a super channel by wavelength defrag, output optical power to an optical transmission line fluctuates and thus signal quality is degraded.